Gas and odour treatment

ABSTRACT

The present invention relates to a product for, and a method of, treating gases, fumes and vapors with fragments of crustacean shell so as to extract one or more components/pollutants from the gases.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a 35 U.S.C. § 371 National Phase Applicationof International Application Ser. No. PCT/GB02/03789, filed Aug. 16,2002, which claims priority to Great Britain Patent Application Ser. No.0120085.6, filed Aug. 17, 2001, the disclosures of which areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a product for, and an improved methodof, treating gases, fumes and vapors so as to extract componentstherefrom, and is of particular but not exclusive use in the treatmentof gaseous effluent from industrial/agricultural processes.

BACKGROUND OF THE INVENTION

Cleaning or decontaminating of effluent gases, as opposed to cleaning orcontaminating effluent liquids present a unique problem to theenvironment.

A variety of bioadsorbents have been employed over the years for thetreatment of gases to either detoxify or reduce their odor. The types ofmolecules removed are generally related to the functionalizedbiopolymers from which the bioadsorbent is composed. These bioadsorbantsinclude, cellulose, alginates, chitin, chitosan and carrageenan. Inaddition ‘activated’ microporous materials such as activated carbon,activated silica and molecular sieves have also been used.

A problem associated with the use of purified extracts for gas treatmentlies not only in the cost of extraction but in their efficacy.

It is known from the prior art to “scrub” volatile zinc fumes producedfrom hot dip galvanizing plants by using crushed limestone. The crushedlimestone is allowed to contact the zinc fumes in a filter bagarrangement so as to bind the zinc, thereafter the bound zinc/limestoneis disposed of However, a problem with this method of scrubbing metalfumes is that not all of the metal becomes bound to the limestone.Moreover the bound metal is not easily recoverable from the limestoneand the bound product in itself is difficult to dispose of.

The present invention provides an alternative bioadsorbant material anda method of producing and using it which is not only more cost effectivethan prior art material but is more efficient at extractingcomponent(s)/pollutants from the fumes/gas vapors.

STATEMENTS OF INVENTION

The present invention resides in the observation that un-denaturedcrustacean shell fragments are efficient at treating gas effluents so asto remove/bind component(s) pollutant(s) from a gas flow.

According to the present invention there is provided a method ofpreparing shells for the treatment of a gaseous composition to extracttherefrom one or more components thereof, the method comprising thesteps of:

-   -   (i) clearing a crustacean shell so as to be free of soft tissue;    -   (ii) fragmenting the shell; by means of a high shear shredder        milling;    -   (iii) sieving the fragmented shell and;    -   (iv) placing the sieved fragments in a vessel through which a        gaseous composition is able to flow, and wherein the sieved        fragments are either static or are fluidized in the gas flow.

Preferably, fragmentation of the shell is by means of a high sheafshredder milling.

Preparation of the shell for use in the method of the invention is amulti-stage process. Preferably, the whole shell is cleaned free fromadherent biological soft tissue prior to fragmentation. This may beachieved preferably by high pressure hosing followed by ultrasoniccleaning for example for about 30 minutes. Preferably the whole shell isthen washed with fresh water and subsequently air-dried at a temperaturewhich does not denature the biochemistry of the shell.

It may also be applicable to use a chemical method of clearing the shellof soft tissue as long as the shell remains un-denatured by such atreatment step.

Preferably, the shell is then fragmented, for example, by means of ahigh shear shredder. Typically each fragment will have front and rearsurfaces derived from the largest outer and inner surfaces of the shellfrom which it formed a part, interconnected by side surfaces of smallersurface area. Preferably the surface area of the largest side of eachfragment is from 10 to 20 mm² More preferably, the surface area of thelargest side of each fragment is about 15 mm².

In one embodiment of the invention the shell is frozen. The frozen shellfragments are then passed through a high shear mill or a pin mill(cryogenic milling).

We have found that cyogenic milling does not have any deleterious effecton the biochemical and/or physical properties of the shell.

Preferably, milled powder is sieved through a screen suitably fittedwith a pore size in the range of 100 to 600 μm, and ideally is 120 to550 μm and most ideally is 480 μm. Larger pieces excluded by the screenmay be re-milled to the correct particle size range and re-sieved. Thesize of the shell particles ideally makes a fine powder of 120 to 480 μmwith a high surface area to volume ratio which improves the efficacy ofthe extraction process. We have found that particle size is an importantdeterminant of efficiency. Whilst it is desirable to have a largesurface area to volume ratio for the particles, we have found thatparticles below a certain size surprisingly loose their ability toabsorb gases presumably due to structural damage during milling. We havealso found that particles above a certain size although maintainingtheir physical integrity have a decreased efficiency due to a lowsurface area to volume ratio.

We have found that crustacean shell is an effective absorbent by virtueof the chemistry/biochemistry present within its structure and that theprinciple mechanism of binding in the absorbent shell fragments ischemisorption.

Preferably, the crustacean shell used in the method of the presentinvention is derived from a crab, prawn, langoustine or lobster and morepreferably the invention involves the use of crab carapace.

Reference herein to fragmentation of the shell is intended to include apowder, part of the shell in particulate form or discrete particlesthereof.

Reference herein to carapace is intended to include part of a wholecarpace, for example, the shell body, arms, legs, claws, tail and/or anyother body part, or portion thereof, of a crustacean from which softtissue can be removed.

Preferably, the crustacean is mature so that, in the instance of using acrab or lobster, at least a part of its carapace is mineralized withcalcium carbonate deposits. The shells of prawns and langoustines areprimarily composed of chitin and even when they are mature the shellsare not always mineralized. However, mature carapaces from any one ofthe selected crustacean perform the function of the present invention.

Other prior art methods of preparing crustacean shells have employedsteps of subjecting the shell fragments to extended periods of hightemperature for example 12 hours at 160° C. or 1 hr at 200° C. andsimilar conditions to effect desorption. However such a step denaturesthe shell fragments so that it can only, function as a physiosorptivetrap. We have found that un-denatured shell retains chemisorptionproperties which enhance the overall cleaning/extraction performance.

According to a yet further aspect of the invention there is provided aproduct comprising crustacean shell fragments of a pore size up to 500μm for use in treating gaseous compositions so as to remove a componentof pollutant therefrom.

Preferably, the product is produced by the method of the presentinvention.

Preferably, the product is of substantially uniform particle size.

According to a yet further aspect of the invention there is provided anapparatus for the treatment of gaseous effluents comprising:

-   -   (i) a container in which adsorbent material is located the        container being provided with entry and exit ports for the        passage of a gaseous composition therethrough and;    -   (ii) an adsorbent material carrier in the container.

Preferably the adsorbent material is fragmented shell.

Preferably, the adsorbent material carrier is an up-flow column. In thisembodiment of the invention fragmented shell is, suitably containedwithin a packed-bed contactor and effluent gas is forced through thecolumn so that it contacts the fragmented shell (FIGS. 4 and 5).

In another embodiment of the invention the adsorbent material carrier isa bag filter. In this embodiment, the adsorbent material may be coatedon to and trapped on the surface of a coarsely woven porous bag.Effluent gas is forced through the bag filter thereby having contactwith the absorbent material.

In order to maintain efficacy it is preferable that a container in whichthe adsorbent material is located allows maximum opportunity for contactof the gas, for example industrial exhaust gas, with the micro-porousadsorbent, while at the same time maintaining the residence time at aminimum in order to facilitate good flow rates. A packed-bed design ofcontactor or a bag filter will facilitate this.

Preferably the apparatus uses the product of the present invention.

According to a yet further aspect of the invention there is provided amethod of treating a gaseous composition to extract therefrom one ormore components thereof, the method comprising contacting the gaseouscomposition with fragmented shell, or part shell as hereinbeforedefined, of a crustacean, said one or more components having a bindingaffinity with said shell, and separating said shell from said gaseouscomposition.

The flow rates and residence (contact) times are variable and depend onthe industrial process in which the powdered shell is employed, thescale of cleaning/scrubbing of gas effluence required and theconcentration of contaminants within the gaseous effluence.

Preferably, the fragmented shell is prepared as hereinbefore describedand may be used with the apparatus as hereinbefore described.

Preferably, volatile and-fine particulate materials are removed from thegaseous composition by binding to the powdered shell fragments by bothphysiological (physioabsorptive) and chemical (chemisorptive) binding.In this way gaseous effluents may be scrubbed/cleaned of undesirablecontaminants.

According to a further aspect of the invention there is provided use offragmented crustacean shell in removing volatile zinc copper, nickel,cadmium or lead from industrial gaseous effluent.

According to a yet further aspect of the invention there is provided useof fragmented crustacean shell in the removal of acidic or sulphur basedgases from industrial effluent.

Preferably the acidic gases include acetic acid, hydrochloric acid andnitric acid which are by-products of a number of industrial processessuch as galvanization and in the manufacture of printed circuit boards.In addition the use may be for the removal of acidic gases such assulphur dioxide and hydrogen peroxide and sulphur based gases such ascarbonyl sulphate, dimethyl sulphate, carbon disulphide, dimethyldisulphide, dimethyl trisulphide, thiols and mercaptans from a range ofindustrial and also biological anaerobic processes.

Preferably, the fragmented shell is prepared as hereinbefore describedfor the various uses and the shell fragments may be used with theapparatus as hereinbefore described.

The invention will now be described by way of example only withreference to the following Figures wherein:

FIG. 1 illustrates an electron micrograph of a crab carapace;

FIG. 2 represents an alternative view of FIG. 1;

FIG. 3 represents an alternative view of FIGS. 1 and 2;

FIG. 4 represents the apparatus of the present invention in situ forscrubbing gases and colors.

FIG. 5 represents a modifiction of the apparatus illustrated in FIG. 4for contacting carapace with gaseous effluent with improved efficiencyof contact.

DETAILED DESCRIPTION OF TE INVENTION

The high binding capacity of the crab carapace is partly due to theinherently microporous nature of the material (similar to activatedcarbon and silica) and the large surface area to volume ration whichresults. The carapace has a micro-porous “box-girder” form ofconstruction in order to provide maximum mechanical rigidity for minimumdensity (FIGS. 1-3). Conversion of the carapace into an adsorbent isachieved by turning shell, for example by milling, into fragments,thereby providing a further increase in surface area for chemisorptionand physisorption.

This present invention describes the optimum particle size of about 480μm which will enclose pores providing a maximum flow rate of gas whileproviding a retention time optimum to the provision of maximum binding.

The extensive internal surface of the carapace consists of a network oforganic and inorganic components. Organic polymers (chitin and fibrousproteins) are wrapped in globular protein and cemented in inorganiccalcium carbonate (Hegdahl et al 1977 a & b).

The positively-charged amine side-groups of the principally globular butalso fibrous proteins, readily bind a variety of contaminating moleculesfound in industrial exhaust gases. In addition there-will be somephysisorption of molecules by electrostatic, van der Waals' forces.

With reference to FIG. 5 there is shown the apparatus of the presentinvention in block diagram form. The apparatus A comprises a container 1with an inlet port 2 and an outlet port 3 for effluent gas to flowtherethrough in the direction of arrows 5 to 6.

The apparatus further comprises an adsorbent material carrier 7. Thecarrier can be in the form of a packed bed contactor or a filter bagarrangement enclosing powdered crab carapace 8. The container is alsoprovided with a drain 9. In operation, gas effluence is passed to thecontainer via inlet port 3 and into the space 10 defined by walls of theadsorbent material carrier 7. Effluent gas then passes in direction ofarrows 11 and through the carrier 7 so that the gas passes over thepowdered crab carapace contained within walls of the carrier, in thisway materials such as zinc can be trapped on the powder.

With reference to FIG. 4 there is shown the apparatus of the presentinvention used for scrubbing gases and odors.

The technology of the present invention provides numerous advantagesover pre-existing methods of gaseous effluent treatment. It is highlycost effective since the raw material is provided annually as a wasteproduct and actually attracts a cost for disposal in landfill sites ordisposal at sea The subsequent processing into a bioadsorbent iscomparatively inexpensive. It is estimated that the present inventionadvantageously could reduce cost by as much as 20 times the currentpractice cost.

Although the retention capacity of the adsorbent varies depending on themolecule being bound within its structure, the crab shell adsorbent alsohas a very high binding capacity.

Disposal of the spent bioadsorbent used to trap acid gases and odors mayby via landfill or incineration. When subjected to composting or whenploughed into arable land, the biochemical components of the milledcarapace (protein and chitin) denature within a few weeks. However, inthe case of carapace used to trap volatile zinc, the zinc-saturatedcarapace is sent to a zinc smelter/refiner where it is mixed with zincore (zinc sulphide) and where zinc metal is recovered. Calciumcarbonate, the principal mineral component of carapace, isbiocompatible. Depending on the pH of the soil, it will either disappearafter several months or be retained within the soil for more prolongedperiods, where it will then serve as a valuable calcium fertilizer.

It is envisaged that the present invention will have wide-rangingapplications in the treatment of gaseous effluents produced from avariety of industrial/agricultural processes. These include:

(i) Removal of volatile zinc from the gaseous effluent of galvanizingplants, and volative cadium and lead from other industrial processes.

(ii) Removal of acid gases such as acetic, hydrochloric and nitric acidused in a variety of industrial processes such as galvanization andmanufacture of printed circuit boards.

(iii) Removal of acidic gases such as sulphur dioxide carbonyl sulphide,dimethyl sulphide, carbon disulphide, dimethyl disulphide, dimethyltrisulphide and hydrogen sulphide from a range of industrial and alsobiological anaerobic processes.

(iv) Removal of molecules causing malodour and sulphur-based gases suchas thiols and mercaptans.

EXAMPLE 1

Milled crab carapace powder, sieved through a screen with a pore size of120 μm was placed in an adsorbent material carrier as depicted in FIGS.4 and 5. A similar arrangement was provided with conventional powderedlimestone adsorbent material carrier. Each apparatus contained 25 Kg ofadsorbent material and gaseous effluent from a zinc galvanizing plantwas allowed to pass through the apparatus for 177 hours each. This timebeing typical for a production run when the plant is operational.

At the end of the run, samples from each adsorbent were taken and thepercentage of zinc was determined using an inductively coupled plasmaanalytical technique. The comparative results are tabulated below:

Parameter Crab Carapace Limestone Percentage trapped Zinc 27.28% 10.47%Weight of trapped Zinc  6.82 Kg  2.62 Kg Trapping Rate 38.53 gm/hour14.80 gm/hour

The results show that powdered crab carapace is more effective thanconventional powdered limestone at trapping zinc from galvanizing planteffluent.

EXAMPLE 2

In a study with milled powdered crab carapace run over a 10 hourproduction time, the following additional metals were found to betrapped in the carapace bioadsorbent: zinc (11,557 ppm); lead (456 ppm);copper (23.8 ppm); nickel (11.7 ppm) and cadmium (7.2 ppm).

These results show that powdered crab carapace is effective at removinga variety of metals from gaseous effluence.

EXAMPLE 3

Crab carapace, powder of 120 μm (Carasol 120) was found to have asurface area of 9.1 m²/g. Crab carapace powder of 480 μm (Carasol 480)was found to have a surface area of 18.2 m²/g. These results shows thatparticle size of approximately 0.5 mm has a higher surface area tovolume ratio than smaller fine powder and that it is a suitablecandidate for extracting contaminants from effluent gases and may beused in cleaning/purifying gases and industrial effluence. Furtherresults have shown that carapace powder of greater particle size thanCarasol 480 have a reduced surface area to volume ratio which indicatesthat particle size is an important determinant in efficacy.

REFERENCES

Hegdahl, T. Gutstavesen, F. & Silness, J. (1977). The structure andmineralization of the carapace of the Crab (Cancer pagulus) 2. Theexocuticle. Zoologica Scripta Vol. 6. pp 101-105.

1. A method of preparing shells for the treatment of a gaseouscomposition to extract therefrom one or more components thereof, themethod comprising the steps of: (i) clearing a crustacean shell so as tobe free of soft tissue but remaining un-denatured; (ii) fragmenting theshell to produce a powder; (iii) sieving the shell powder through ascreen and; (iv) placing the sieved powder in a vessel through which agaseous composition is able to flow, and wherein the sieved powder iseither static or fluidised in the gas flow.
 2. A method according to 1wherein clearing the shell comprises hosing the shell by high pressurefollowed by applying ultrasonic cleaning.
 3. A method according to claim2 wherein the ultrasonic cleaning is applied for about 30 minutes.
 4. Amethod according to claim 1, wherein step (i) further comprises washingthe whole shell with water and subsequently air-drying the shell.
 5. Amethod according to claim 1, wherein the shell powder produced by step(ii) results in particles with a surface area of about 10 to 20 mm². 6.A method according to claim 5 wherein the particles have a surface areaof about 15 mm².
 7. A method according to claim 1, further comprisingsieving the powder through a screen with a pore size in the range 100 to600 μm.
 8. A method according to claim 7 wherein the pore size is in therange 120 to 550 μm.
 9. A method according to claim 7, wherein the poresize is approximately 480 μm.
 10. A method according to claim 1, whereinthe crustacean is selected from the group comprising crab, prawn,langoustine or lobster.
 11. A method according to claim 1, furthercomprising freezing the shell prior to fragmentation and then millingthe shell under cryogenic conditions.
 12. A product comprising powderedUn-denatured crustacean shells of pore size up to 500 μm for use intreating gaseous compositions so as to extract a component or pollutanttherefrom.
 13. A method of treating a gaseous composition to extracttherefrom one or more components thereof, the method comprisingcontacting the gaseous composition with powdered un-denatured shell, orpart shell, of a crustacean, said one or more components having abinding affinity with said powdered shell, and separating said powderedshell from said gaseous composition.
 14. A method of removing volatilezinc, copper, nickel, cadmium or lead from industrial gaseous effluentthrough a powdered un-denatured crustacean shell.
 15. A method ofremoving acidic gases or sulphur based gases from industrial effluencethrough a powdered un-denatured crustacean shell.
 16. The methodaccording to claim 14 wherein the powdered un-denatured crustacean shellis prepared by clearing a crustacean shell so as to be free of softtissue but remaining un-denatured, fragmenting the shell to produce apowder, sieving the powder, and placing the sieved powder in a vesselthrough which a gaseous composition is able to flow, and wherein thesieved powder is either static or fluidized in the gas flow.
 17. Themethod according to claim 15 wherein the powdered un-denaturedcrustacean shell is prepared by clearing a crustacean shell so as to befree of soft tissue but remaining un-denatured, fragmenting the shell toproduce a powder, sieving the powder, and placing the sieved powder in avessel through which a gaseous composition is able to flow, and whereinthe sieved powder is either static or fluidized in the gas flow.